Alkaline batteries based on manganese dioxide cathodes and zinc anodes are widely used for consumer portable electronic applications. There is a large market for primary alkaline cells in standard cylindrical formats such as AAA, AA, C and D sizes. These products have numerous advantages. Zinc and manganese dioxide are inexpensive, safe and environmentally benign and the system provides good energy density. For the consumer, these standard alkaline products have long offered a simple and convenient universal solution for an array of electronic products.
There has been a proliferation in recent years, however, of new portable electronic devices including personal digital assistants, MP3 recorders and players, DVD players, digital cameras, or the like. There is also a trend toward smaller and lighter portable electronic devices that limit the onboard battery size. Compared to earlier devices, such as, for example, transistor radios, the power consumption for many of these new devices can require higher continuous or pulse currents. Conventional or even premium alkaline cell designs cannot efficiently deliver their stored energy at the higher drain rates.
FIG. 1 (section A) shows the capacity that can be delivered by a premium commercial alkaline AA cell under five discharge conditions intended to simulate various consumer electronics application loads (based on American National Standards Institute tests, Reference ANSI C18.1M, Part1-2001). At low drain rates (radio/43 ohm discharge) the alkaline AA “bobbin” cell delivers nearly all of its theoretical capacity (about 3 Ah); at intermediate loads (electronic game/250 mA discharge, motorized toy/3.9 ohm discharge) about two-thirds of theoretical; and at moderately high to high drain rates (photoflash/l Amp pulse, digital camera/l Amp continuous discharge), only ¼ to ½ of theoretical capacity can be accessed.
These inefficiencies under high rate discharge are related to internal resistance and electrochemical limitations of the conventional alkaline bobbin-cell construction. While much effort has gone into improving the energy content of the conventional alkaline bobbin cell by optimizing the internal packing and ionic conductivity of the electrodes, the fundamental design itself has changed little.
As shown in FIG. 2, a typical alkaline manganese dioxide-zinc bobbin cell 10 comprises the following main units: a steel can 12, optionally coated with a conductive coating on the inside of the can, defining a cylindrical inner space, a manganese dioxide cathode 14 formed by a plurality of hollow cylindrical pellets 16 pressed in the can, a zinc anode 18 made of an anode gel and arranged in the hollow interior of the cathode 14, and a cylindrical separator 20 separating the anode 18 from the cathode 14. The ionic conductivity between the anode and the cathode is provided by the presence of potassium hydroxide, KOH, electrolyte added into the cell in a predetermined quantity.
The can 12 is closed at the bottom, and it has a central circular pip 22 serving as the positive terminal. The upper end of the can 12 is hermetically sealed by a cell closure assembly which comprises a negative cap 24 formed by a thin metal sheet, a current collector nail 26 attached to the negative cap 24 and penetrating deeply into the anode gel to provide electrical contact with the anode, and a plastic top 28 electrically insulating the negative cap 24 from the can 12 and separating gas spaces formed beyond the cathode and anode structures, respectively. The material of separator 20 may consist of laminated or composite materials or combinations thereof. Typically separator materials comprise an absorbent fibrous sheet material wettable by the electrolyte, and an insulating material being impermeable to small particles but retaining ionic permeability.
While the bobbin cell construction is a simple design that allows for high-speed, low-cost manufacturing, the surface area between the anode and cathode in a conventional bobbin cell is limited to the geometrical surface area of the cylinder of separator between the anode and cathode. Thus, for a bobbin cell, the anode to cathode interfacial surface area (Si) constituted by the interposed straight cylinder of separator is necessarily a fraction of the external surface area (Se) formed by the cylindrical wall of the can [(Si)/(Se)<1].
In the field of batteries, the surface area of—and between—the electrodes of an electrochemical cell is understood to be an important design element, since the mass transport flux of ions between anode and cathode (typically slower than electron transfer or chemical kinetics) can be a rate limiting or current limiting physical process. It is not only the ionic conductivity and surface area between the anode and cathode that is important but also the micro-porosity and surface area inside the electrodes.
It is possible to arrange for greater electrode and interfacial area within a cylindrical cell. The most widely used cylindrical cell design alternative to the bobbin cell is the spirally wound or jelly-roll construction which is well described in the Handbook of Batteries [3rd Edition, editors D. Linden and T. B. Reddy, Section 3.2.11, McGraw-Hill, 2002]. In this construction thin strips of anode and cathode with separator between them are tightly wound together. The electrodes can be as thin as a few tenths of a millimeter and for the spirally wound cylindrical cell the anode to cathode interfacial surface area can be several multiples of the external surface area formed by the cylindrical wall of the can [(Si)/(Se)>>1]. The greater interfacial area comes at the expense of additional complexity and cost to manufacture. Spiral winding requires precision alignment of anode, cathode, and separator, with lower production rates and higher capital equipment costs than “bobbin” construction cells. The spirally wound design is not typically applied to the alkaline MnO2/Zn cell where it would defeat the economic advantage of the materials, but is applied to more premium electrochemical systems including rechargeable nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries, and non-rechargeable systems such as lithium iron disulfide (LiFeS2) batteries.
Another trade-off of the spiral wound design is the higher amount of separator and current collector required, which take up volume that could otherwise be utilized for active material. Since a standard size cylindrical cell has a fixed volume, it is most efficiently built with maximum active material and electrolyte in order to maximize its energy content. In the bobbin cell, in addition to lower separator content and thick electrodes, the brass nail anode current collector and cathode current collection via contact with the cylindrical container wall do not significantly intrude on the interior space.
Thus, while converting from a bobbin design to spiral wound design increases the inter-electrode surface area and power capability, it also reduces the energy content of the cell. A spiral wound construction may deliver most of its energy efficiently for discharge rates on the order of 20 C(C refers to a current equivalent to the rated capacity of the cell in ampere-hours divided by 1 hour). Such high rate discharge capability may be essential for applications such as power tools, however is not typically needed for consumer electronics. Even devices such as digital cameras typically operate at more moderate discharge rates on the order of ⅓ to 1C rate.
More costly spirally wound batteries may be over designed for many portable applications. However, for alkaline manganese dioxide cells with a zinc anode and potassium hydroxide electrolyte to maintain their competitive advantage as a universal solution for a wide range of consumer applications, better run time at higher drain rates is needed. Much of the recent patent literature related to the alkaline cell is aimed at addressing this issue.
In addition to material and electrode formulation strategies to improve power capability, there have been a number of strategies to increase the interfacial surface area between the anode and cathode through modifications of the conventional bobbin cell. For example, Urry in U.S. Pat. No. 5,948,561 describes the use of a bisecting conductive plate coated with cathode active material to partition a V-folded tubular separator. Luo et al. in U.S. Pat. No. 6,261,717 and Treger et al. in U.S. Pat. No. 6,514,637 also describe the creation of multiple anode cavities that are in these cases molded into the cathode pellets. Getz in U.S. Pat. No. 6,326,102 describes a relatively more complex assembly with two separate zinc anode structures in contact with the inner and outer contours of separator encased cathode pellets. Jurca in U.S. Pat. No. 6,074,781 and Shelekhin et al. in U.S. Pat. No. 6,482,543 describe stepped interior or contoured interior surfaces of the cathode pellet. Shelekhin et al. in U.S. Pat. No. 6,482,543, Lee et al in U.S. Pat. No. 6,472,099 and Luo et al. in U.S. Pat. No. 6,410,187 describe branched or lobed interior electrode structures.
All of these design strategies have limitations in the effective increase in surface area that is possible and introduce additional complexities that detract from the utilitarian design of the conventional bobbin cell. Some may achieve greater surface area but at the sacrifice of a cell balance change that decreases the energy content. Multi-cavity or multiple electrode designs introduce the need for more complex current collection and end seals. The more complex geometries may introduce orientation requirements and the need for more complex tooling and machinery for assembly. Complex geometries can make it difficult to apply separator uniformly and consistently especially in high-speed production, and may necessitate unconventional approaches such as internally applied conformal coatings.
For example, branched or lobed designs have limited ability to increase surface area unless the lobes are made thinner which makes applying separator and filling uniformly with gelled anode more difficult. If the lobes or branches are not thinner and longer then not much increase in surface is provided and the cell balance may be changed to be less efficient due to changes in relative cross-sectional area of the anode and cathode structures. Alignment of cathode pellets and breakage of pellets in lobed designs could make manufacture difficult.
Among other things, the present invention is intended to address these as well as other shortcomings in the prior art.